1. Field of the Invention
The present invention relates to a method and apparatus for manufacturing a progressive multi-focal lens for visual acuity correction, and to a progressive multi-focal lens and a spectacle lens manufactured by the method.
2. Description of Related Art
A progressive multi-focal lens is a lens which has two distinctive visual-field portions with different refractive powers and a visual-field portion therebetween in which the refractive power progressively changes. There is no border separating these visual-field portions, which makes their appearance excellent. Further, it is possible to obtain other visual fields of different refractive power in one lens. For these reasons, a spectacle lens is used which has a function of correcting visual acuity such as presbyopia. In many progressive multi-focal lenses, a distance portion which is a visual-fieled portion for seeing objects at long distances is arranged in an upper position, and a near portion, which is a visual-field portion which has different refractive power from the distance portion for seeing objects at short distances, is arranged below the distance portion. Furthermore, these distance and near portions are smoothly connected by a progressive portion, which is a visual-field portion with a refractive power which continually changes in order to see objects at intermediate distances between the long and short distances. Therefore, even a user whose eyesight adjustment power has deteriorated due to progressive presbyopia can gain a clear visual field, focusing at both far and near distances by using spectacles which use progressive multi-focal lenses. Moreover, as for the distance between near and distance portions, because the refractive power progressively changes, it is possible to gain a clear visual field even at an intermediate distance. Therefore, progressive multi-focal lenses are favorably used by many users as spectacle lenses and it is conceivable that they will be used by more users for different purposes in the future.
A progressive multi-focal lens has at least two areas with different refractive powers and a progressive surface which continuously changes between these two areas through a progressive portion. Therefore, since this progressive portion is not simply a spherical surface, the curvature of at least one area of progressive surface is different in two orthogonal directions and because of this, astigmatic aberration occurs. However, for example, for a user with spectacles without astigmatism, if the astigmatic aberration which appears in a lens is 1.0 diopter or less, preferably 0.5 diopters or less, it is possible to obtain a clear vision with little perceptible blurring of the image. Therefore, for a driver or a user who likes sports, who often sees far, a progressive multi-focal lens in which a wide clear vision area to obtain clear vision in a distance portion is preferable. Meanwhile, for a clerical worker or a user whose hobby is knitting or the like, who often sees near, it is preferable to use a progressive multi-focal lens in which a wide clear vision area is in a near portion. As described earlier, in a progressive multi-focal lens, lens power suited to a visual acuity of the user is needed, and further, it is preferable to change the lens power depending upon the purpose of the spectacles.
Furthermore, for the movement of a person's eyes from a far distance to a near distance, the movement of the eyeballs converges toward the nose. Because of this, it is preferable for the main focus visual line which extends from the distance portion to the near portion to curve this convergence by adjusting convergence of the eye slightly toward the nose and to arrange a clear vision area along the main visual line. Because the amount of the convergence varies depending upon the user, it is not possible to sufficiently obtain a clear visual field unless the progressive multi-focal lens has a clear vision area which is suitable to the user. Other than that, several elements exist which affect matching between a progressive multi-focal lens and the user, such as a rotation angle and a progressive band.
As described earlier, if a user tries to obtain a comfortable visual field, it is necessary to select a progressive multi-focal lens after considering the above points.
Conventionally, as shown in FIG. 14, a basic progressive multi-focal lens system is used which has progressive surfaces with several kinds of average optical characteristics formed in a convex side that are prepared by a manufacturer, and from among these, a lens which is considered to be suitable to the user is selected, and in which the concave side is processed depending upon the diopter of the user. A schematic process up to the time when the progressive multi-focal lens is shipped to market is explained as follows: First of all, in step 91, a manufacturer establishes a progressive surface which is suitable to several kinds of lenses, for example, a lens focused on far and intermediate distances, a lens focused on near and intermediate distances, and further, a lens balanced for far, intermediate, and near distances, depending upon an average purpose of the user, a movement of his eye, and the like, and a lens of a state in which the progressive surface is formed on a convex side is processed. Furthermore, in step 92, an inventory of processed semi-finished lenses is prepared. As shown in step 93, the user obtains information concerning his/her eyes such as the diopter, astigmatism, convergence, rotation angle, and the like in a spectacles store, or the like, determines the purpose of the spectacles, and selects lenses which he thinks are the most appropriate from among the semi-finished lenses. The task of selecting appropriate lenses from among the semi-finished lenses often requires professional knowledge, so this work is normally performed by a store or a manufacturer, and there are cases when software is used for selection.
In step 95, the selected concave surface of the lenses is processed to the diopter which is suitable to the user including the astigmatism correction, and the lenses are shipped to market in step 96. During processing, processing shape is determined in response to the spectacles frame which the user has selected, and further, additional work such as color coating is performed, if requested. After passing through processing such as this, a lens which is suitable to the user is selected from among the manufacturing group which the manufacturer prepared, and delivered to the user. Furthermore, as user information which is obtained in step 93 is more thoughtfully considered, a lens which can obtain a comfortable vision field which meets the needs of the user can be provided from among the manufacturing group prepared in a semi-finished state.
In order to select a product which is suitable to a user, semi-finished lenses need to be stored in stock which can meet the needs of the user. However, kinds which can be manufactured in a semi-finished state are limited in order to keep the number or volume to be stored in an appropriate scope, and are limited to several kinds such as the far-use focused outdoor type, far-, intermediate-, near-balanced for the wide-use type, and the intermediate- and near-focused indoor type which are average and are in great demand. Therefore, a user can only select the most appropriate lens from among the kinds of progressive multi-focal lenses which are already prepared and cannot always wear the most appropriate lenses for himself.